1. Field of the Invention
This present invention relates to apparatus used to retrieve downhole samples of reservoir fluids from earth bores. With more particularity, the present invention relates to apparatus used to secure subsurface samples of fluids at a pressure at least as great as that of the reservoir pressure at the level at which the sample was found. With further particularity, the present invention relates to sampling apparatus designed to lower downhole, capture a pressurized fluid sample downhole and maintain at least downhole pressure of said sample as the sampling apparatus (and sample therein) is subjected to cooling during retrieval and transfer to pressurized laboratory testing or storage device.
2. Description of the Related Art
Petroleum reservoir fluids, particularly oils, vary greatly in their physical properties from reservoir to reservoir. Properties such as composition, viscosity, gaseous phase envelope and solid phase envelope greatly affect the potential value of a reservoir. These properties affect whether production may be economically achieved at all and, if so, the duration, expense and per unit price of said production. For this reason securing very accurate oil samples, for detailed testing, is of key importance.
Various methods exist to take oil samples. One method is to take samples of the produced oil and gas streams at the surface, combine the oil and gas in a manner believed to create a recombinant sample as it exists in the reservoir, and perform tests on the recombinant. Petroleum reservoirs are usually several thousands of feet from the earth's surface and typically have pressures of several thousands of pounds per square inch and temperatures on the order of 250 degrees Fahrenheit or more. However, substantial inaccuracies may occur in testing of recombined samples, as several, irreversible changes may have already occurred during flow of the downhole fluid to the surface. During flow of downhole fluid to the surface both pressure and temperature drop dramatically. Such changes may cause certain components of the downhole fluid to irreversibly precipitate from solution and/or colloidal suspension and thereby be underestimated from surface sampling. Such downhole changes, such as paraffin or asphaltene deposition, could nevertheless be causing substantial downhole damages to the well. Such damage might have been entirely avoidable, if accurate testing had shown the precise composition, pressure and temperature of their formation.
An improvement is subsurface sampling. While subsurface oil sampling is preferred so as to secure a more representative sample of downhole composition and thereby increase the accuracy of the test results, preventing irreversible changes in the work sample during retrieval to surface and discharge into pressurized test or storage devices has remained problematic. Early sample tools employed a fixed volume, initially evacuated chamber, that was lowered to the formation desired to be sampled, where a valve was opened to allow inflow of oil into the chamber. Once filled, the valve was closed, retaining the sample, and the chamber was brought to the surface. During retrieval of the sample tool to the surface, cooling of the sample, in a fixed volume, resulted in a sample pressure decrease. Decreased pressure often resulted in gasification of certain fractional components as well as irreversible precipitation of certain solid components. While very careful laboratory studies could be conducted on at least a partially recombined sample, and further testing could be performed on components irreversibly separated from the original sample, there persisted a margin of possible inaccuracy which was sometimes critical to very valuable producing properties. As those skilled in the art know some producing properties can be problematic and expensive to shut-in for cleaning or reworking and may be difficult, if not impossible, to restore to production following rework.
Efforts to limit or prevent phase change of samples during retrieval and transport to laboratory or pressurized storage devices have resulted in variable-volume sample chamber tools of two broad groups:
A. Tools having a sample chamber made variable in volume by inclusion of an internal reservoir of elastic volume therein. PA1 B. Tools having a sampling chamber made variable in volume by means of a pressurized incompressible fluid. An elastic means, such as gas or a spring, is typically used to pressurize said incompressible fluid, either directly or through a second piston.
Whitten, U.S. Pat. No. 3,859,850 (Jan. 14, 1975), Bimond, et al, GB 2012722 A (1979), Petermann, U.S. Pat. No. 4,766,955 (Aug. 30, 1988), and Gruber, et al, U.S. Pat. No. 5,009,100 (Apr. 23, 1991), all disclose subsurface sample tools that employ a sample chamber of the nature of tools described in group (A) above. A reservoir of trapped gas is included in the sample chamber. The volume of said reservoir is, essentially, made elastic by means of a piston which may be compressed internally (when pressure outside of the reservoir is greater than internal pressure of the reservoir). As the sample tool is lowered downhole the reservoir of trapped gas, if lower in pressure than downhole pressure, decreases in volume (a piston in the reservoir is forced inward). In theory, on cooling and contraction of the sample (as by retrieval to the surface), the gas in the reservoir will re-expand and maintain pressure of the sample. However, in order for the volume of the reservoir of trapped gas to contract upon descent downhole (and therefore be capable of re-expanding on retrieval) its initial pressure must be something less than bottom hole pressure of the sample. Additionally, as the sample cools on retrieval, so does the trapped gas, further reducing the ability of the trapped gas to re-expand fully from downhole conditions. Thus, while tools of group (A) may be of some utility, at least for the purpose of limiting the amount of pressure losses in a fluid upon retrieval from downhole, they are inherently incapable of maintaining the sample at or above downhole pressure condition during retrieval. Such tools also fail to disclose leakproof piston seal design, and the possibility of gas leakage is mentioned in Bimond et al. In order to detect and account for such leakage Bimond et al. teaches the use of a tracer gas, such as carbon tetrafluoride, which is not found in the sample.
As alternatives to the tools of group (A) are tools of group (B) such as McConnachie, GB 2022554 A and Massie, et al, U.S. Pat. No. 5,337,822 (Aug. 16, 1994). These tools represent an improvement to the tools of group (A) in the sense that both have the capability of retrieving a sample while maintaining a sample pressure at or above original down hole pressure. Despite at least the possibility of improved performance both tools, however, utilize an incompressible fluid to drive, either directly or indirectly, against a trapped volume of sampled fluid. A piston is utilized to pressurize the incompressible fluid. Said piston is powered by an elastic source such as a gas or a spring.
McConnachie, GB 2022554 A, discloses a subsurface flow-through sampling tool. As the sampler descends in the well, oil enters and exits the sample chamber through flow-through ports. Once at the desired depth, oil is trapped in the tool by a sliding dual piston means. Valve means then releases a pressurized gas, driving a piston that displaces mercury under pressure into the sample chamber. The resulting sequence of pressure transmission forces to maintain pressure on the sample is: pressurized gas--piston--mercury--oil sample.
Massie, et al, U.S. Pat. No. 5,337,822 (Aug. 16, 1994) employs a sample chamber divided by a movable piston. Said piston is pressurized against the sample by an incompressible fluid such as mineral oil. The mineral oil is, in turn, pressurized by a movable piston contained in a second chamber. The movable piston of the second chamber is, in turn, driven by an elastic source, such as a spring or a gas in said second chamber. The resulting sequence of pressure transmission forces to maintain pressure on the sample is: elastic source--second piston--incompressible fluid--first piston--oil sample. The Massie tool employs numerous parts and relies on a lengthy sequential operation of multiple valves with the attendant chance of malfunction. Accordingly each of the aforesaid sampling tools designs are either limited in performance or inherently complex, costly, likely to require substantial maintenance and/or are prone to malfunction.
It is therefore the principal object of the present invention to provide an improved tool for taking of downhole samples of fluids in an earth bore. A particular object of the invention is to provide a downhole sampling tool capable of maintaining pressure of the sample at, or above, downhole pressure during retrieval of the sample to surface, despite thermal contraction of the sample by virtue of cooling upon retrieval. With greater particularly an object of the present invention is to provide a downhole sampling tool of simple, efficient, reliable and inexpensive design characteristics. Specific objects of the invention are to provide a downhole sampling tool accomplishing the above listed objectives, which further embodies use of only one piston in its pressure charging circuit; eliminates the use of a piston-incompressible fluid-piston segment in its pressure charging circuit; has a simplified valve means which switches from a filling mode to a pressurization mode by shifting of a single control spool; and has a minimal number of movable parts cooperating in a simple operational sequence to delay filling until downhole, automatically fill with sample when desired, and maintain the pressure of said sample at or above downhole pressure despite thermal contraction of the sample on retrieval to the surface.